1. Field of the Invention
The invention related to a system for producing a correction signal for the aperture correction of video signals.
2. Description of the Prior Art
Aperture correction of video signals comprises correction of the distortion produced by the finite dimension and non-uniform flux distribution of the scanning point, in the camera as well as in the reproduction screen. Thus, for example, the frequency spectrum of the video signals obtained by means of known camera tubes falls off more or less intensely toward the high frequencies, still within the bandwidth of 6 MHz. Such an aperture distortion, caused by amplitude distortion, causes a symmetrical obliteration of the pulse edges in the video signal. For the elimination of such aperture distortions, horizontal and vertical aperture correctors are used to reamplify the higher frequencies of the video signal.
Devices for amplifying the higher frequencies of the video signal in aperture correctors are known from Dillenburger, Einfuhrung in die Fernsehtechnik (Introduction to Television Technology), Volume 2, pages 312 to 324. These known aperture correctors, however, are dependent on the bandwidth of the video signal to be corrected. Thus, a video signal to be corrected, with pulses of differing rise time, will itself only be differentially corrected. Pulses with a short rise time are given a steeper pulse-edge slope than are pulses with a long rise time.
A picture is considered distinct when some sharply defined edges are contained in the reproduced image. Since known aperture correctors supply correction signals whose amplitude depends upon the rise time of the black-to white-amplitude changes contained in the video signal, the pulses of a video signal in whose picture patterns there are only long rise times are reproduced in an entirely uncorrected or only slightly corrected manner. The reproduced picture appears blurred. Assuming that during the time period of a picture-scanning at least one black-to white-amplitude change is contained in the video signal, a reference value can be derived for other signal amplitudes in the video signal by means of this black-to white-amplitude change for one or more partial pictures.